Processes for the preparation of oligosulfanes have long been known. Various known systems are described below.
DE-PS 2141159 (U.S. Pat. No. 3,842,111) describes a process for the preparation of bis-(alkoxysilylalkyl) oligosulfanes from the corresponding halogen alkylalkoxysilane and alkali metal oligosulfides, preferably in alcoholic solution. However, only mixtures of sulfanes with different chain lengths are produced. These patent documents are entirely incorporated herein by reference.
In a process described in DE-PS 2712866 (U.S. Pat. No. 4,129,585), an alkali metal alcoholate is reacted with a halogen alkylalkoxysilane, metal or ammonium hydrogen sulfide, and sulfur in the presence of an organic solvent. However, the preparation of the alkali metal alcoholate solution is very time-consuming, which makes industrial-scale use of this process problematic. These patent documents also are entirely incorporated herein by reference.
U.S. Pat. No. 5,466,848 discloses a process in which hydrogen sulfide is reacted with an alkoxide, and the reaction product is treated with elemental sulfur, and then with a halogen alkylalkoxysilane, to give the desired organosilicon polysulfane. Likewise, the process according to U.S. Pat. No. 5,489,701 involves working with alkoxides and hydrogen sulfide, a compound which is known to be very unpleasant to handle. JP-OS 7-228588 describes the reaction of anhydrous sodium sulfide and sulfur with halogen alkoxysilanes. This procedure gives a mixture of polysulfanes, as experiments have shown. These three patent documents also are entirely incorporated herein by reference.
Organosilicon polysulfanes, and especially bis(triethoxysilylpropyl)tetrasulfane, are used in combination with highly active silicic acid fillers in the manufacture of vulcanized rubber articles, especially tires.
The advantageous use of high purity disulfanes in terms of both the processing of the material, and of the resulting properties of the vulcanizates, is set out in EP-A 0 732 362 (U.S. Pat. No. 5,580,919) and by Panzer (L. Panzer, American Chem. Soc., Rubber Div. Meeting, 1997). These documents also are entirely incorporated herein by reference.
The Na.sub.2 S.sub.x required for disulfane preparation can be obtained by reacting sulfur and sodium in molten form, as described in DE-PS 38 03 243, which document is entirely incorporated herein by reference. It should be noted, however, that the melting point of the polysulfide is inversely proportional to its sulfur content. Thus, for example, the Na.sub.2 S.sub.2 required to produce oligosulfane mixtures of high disulfane content has a melting point of 474.degree. C. The only suitable reactor material for such a high temperature is graphite. The difficulties of obtaining reactor components in the necessary dimensions makes industrial application of this process impractical.